Geotechnical and risk analyses for the positioning of shafts at the Wesizwe Platinum project

Abstract

The Wesizwe main and ventilation shaft positions are sited within an almost square block formed by four faults. The shaft positions were not sited within the centre of gravity of the Wesizwe lease area. This is due to factors related to the local community and various environmental issues. It was decided to position the shaft in the current block as geological and rock engineering confidence was high in regard to the structures within the area, and the shaft position was falling within the allowable distance radius from the nearby community and river boundary. A seismic survey, conducted on the area, indicated a near vertical fault. The fault, which has a 30m throw, occurs approximately 50m north of the position at which the main shaft was site. It was suggested that this fault, which was a reinterpretation of the northern boundary fault, would have a negative effect on the rock mass behaviour in the shafts. Previously it was noted that the shaft level breakaways and geotechnical borehole information do not correlate. A 20m vertical discrepancy was observed between the planned Merensky breakaway and the Merensky position indicated by drilling. The logical interpretation was that this was due to faulting and a note was sent out to make the project team aware of this 20m discrepancy. This was again brought to the team’s attention upon the release of the seismic study interpretation indicating a 30m fault in this area. Geotechnical logging had already been done on the diamond drill holes sunk at the positions indicated for the Wesizwe Main and Vent Shafts. The rock mass ratings indicated that these positions were favourable and that the rock mass of the shafts can be referred to as “Good Rock”. To determine what the geotechnical character of the fault was, four boreholes (WF01, WF049, WF059 and WF090) were geotechnically logged at the predicted depths of the fault intersection. Two zones carrying less competent ground were identified near the fault intersection positions as was indicated by the seismic interpretation. By combining the zones into one area of less competent ground it was found that the affected area does not exceed a vertical influence of 58m, and has a minimum vertical influence of 25m. The rock mass in these affected areas are overall rated as “poor rock” to “exceptionally poor rock”. The rock quality designation (RQD) ratings for the affected area fall between 36 to 52 %. A decision needed to be reached as to whether the shafts would be developed at the positions indicated, or whether a new area needed to be selected for the main and ventilation shaft positions. This decision was reached by combining all available information and weighing the risks related to the options. From this study, a general approach to shaft positioning for platinum projects was formulated.